The present invention relates to circuit boards, and more particularly to a novel process for producing circuit boards in which a conductive layer of metal forms a circuit pattern upon, and is firmly bonded to, a resinous substrate providing the support therefor, and to the well defined and bonded circuit boards which are produced thereby.
Various techniques have been utilized for generating printed circuit boards, including the lamination of foil to resinous or ceramic substrates and etching of the circuit pattern, the chemical deposition of the metal pattern from a plating solution, and the deposition of a conductive metal layer or pattern by vacuum metallization, sputtering, and like techniques, followed by electrodeposition of the thicker metal deposit of the circuit pattern.
Traditional copper-clad boards are manufactured using multiple layers of epoxy resin impregnated glass cloth which are pressed under heat and pressure together with copper foil to form a copper-clad laminate. To enhance the adhesion of the epoxy/glass prepreg to the copper, the contact surface of the copper is treated so as to form a rough oxide surface. The ultimate peel strength is defined both by mechanical adhesion of the reflowing epoxy during the pressing phase as it flows into the microrough oxide surface of the copper foil and also by the chemical adhesion of the epoxy resin to the copper oxide. These copper-clad boards are then used for the manufacture of printed circuit boards using subtractive processing, i.e., the removal of the copper between the desired circuit elements of the conductive pattern.
In an effort to obtain fine patterns with well defined edges, industry has been decreasing the thickness of the metal layer, and problems of obtaining good bond between the metal layer and the underlying substrate have been increasing. Thus, where line widths and spacings were commonly 0.008-0.015 inch several years ago, currently demands are being made for lines width and spacings of 0.003-0.007 inch and sometimes as little as 0.001 inch.
The smaller the width and the spacing, the thinner the metal layer must be for sharp definition. As is known, the thicker the copper deposit to be etched, the greater the extent of the etch-back. Therefore, to achieve fine lines, it is necessary to have a thin layer of copper to etch; the finer the desired lines, the thinner the copper. The production of foil laminated boards using an extremely thin copper layer necessary is impractical so that industry has turned to additive and semi-additive processing, i.e., the deposition of metal onto the substrate.
The main problem in such additive processing is one of the adhesion of the copper layer deposited onto a cured epoxy laminate. Because adhesion is not reversible, the peel strength will not be the same as that achieved in a foil/board laminate.
Many techniques and methods have been tried to overcome this deficiency, but none has fully overcome it in a practical, production-efficient way. Among the techniques which have been proposed to improve the bonding or peel strength between the metal layer and the underlying substrate are application of intermediate "adhesive" coatings and etching or roughening of the substrate before application of the metal layer, etc. Generally, these techniques have effected improvements in bond strength, but they have not produced uniformly good bonding and/or have involved substantial additional expense and processing difficulties.
It is an object of the present invention to provide a novel additive process for the production of circuit boards in which a thin metal layer is deposited upon and firmly bonded to an underlying resinous substrate.
It is also an object to provide such a process which inherently lends itself to the fabrication of miniaturized circuitry with good adhesion of the individual circuit portions to the underlying substrate.
Another object is to provide such a process which may be practiced relatively economically and expeditiously.
A further object is to provide circuit boards utilizing relatively thin metal deposits of relatively narrow width and close spacing to form the conductive pattern, and in which the conductive pattern is firmly bonded to a resinous substrate.